The panel looks inconsistent with the different icon spacings in the "indicator plugin" and the "notification area". It would be nice if the spacing could be defined at runtime, to adjust it accordingly to the "indicator plugin". The picture at http://i.imgur.com/YtPEQ2O.png shows the different spacings.
The SPACING constant in http://git.xfce.org/xfce/xfce4-panel/tree/plugins/systray/systray-box.c seems to provide the value to define the spacing between icons.
There is now a "square" option in both of those plugins, so it's all more consistent now.